Parliamentary entry on the national level is a crucial achievement for any new party. But its repercussions are not necessarily beneficial. This article assesses the electoral consequences of parliamentary breakthrough by theorizing factors that shape (a) a new party organization’s capacity to cope with pressures generated by parliamentary entry and (b) the relative intensity of the new functional pressures a new party is exposed to after breakthrough. To test our hypotheses derived from these two rationales, we apply multilevel analyses to a new data set covering 135 organizationally new parties that entered national parliament across 17 advanced democracies over nearly five decades (1968–2015). Our findings stress the importance of party organizational characteristics (party origin, time for party building and leadership continuity) for parties’ capacity to sustain electoral support after breakthrough. In contrast, the intensity of functional pressures as generated by government participation immediately after breakthrough does not have significant effects on parties’ performance at the follow-up election.